The present invention relates to a process for recovering the chlorine value from metal chlorides and the conversion of the metal chlorides to metal oxides.
Many industrial processes that are designed to convert mineral ores to products of greater purity and value involve an initial step wherein metal values in the ore are converted to metal chlorides. The process for the production of titanium dioxide pigment, processes for the production of titanium or zirconium metal are examples of such conversion processes where metal values in the ore are first converted to metal chlorides.
The conversion of ore metal values to metal chlorides provides a means to separate iron and other metal chlorides from those of the higher valued metals, such as those of titanium and zirconium; but there continues to be a need for a process by which the chlorine values from the iron and other metals considered to be of low value may be recovered.
Several processes for the recovery of chlorine have been taught, such as those in U.S. Pat. No. 2,642,339 to Sawyer; U.S. Pat. No. 4,994,255 to Hsu; U.S. Pat. No. 3,793,444 to Reeves et al.; U.S. Pat. No. 4,174,381 to Reeves et al. (all fluidized bed processes); U.S. Pat. No. 3,919,400 to Bonsack (reaction of ferric chloride vapors with sodium chloride and oxidation of the salt complex formed with molecular oxygen); and U.S. Pat. No. 4,576,636 to Robinson et al. (oxidation of ferrous chloride with staged additions of oxygen). The practical application of processes such as these have been limited due to the build-up of deposits, particularly hard scale deposits, on the walls of the process reactor.
Some of the patents listed above address the problem of wall build-up in the reactor. For example, those of Sawyer and Hsu each teach the addition of particles to a fluidized bed reactor to scrub the reactor wall. U.S. Pat. No. 3,092,456 to Nelson and U.S. Pat. No. 3,325,252 to Wikswo et al., each teaches the use of a material to cool the reactor wall.
U.S. Pat. No. 3,464,792 to Herriman et al. teaches introducing a second gas stream (which may be the oxidizing gas, metal halide vapors or a mixture of these) through several injection inlets into a first high-temperature, arc-heated gas stream (that may contain oxygen or a mixture of oxygen, metal halide vapors and an inert gas) in such a way that the second gas stream cools the first gas stream and the reactor walls. According to Herriman et al., there must be a uniform flow of the reactants in this process; and therefore, the injection inlets for the two gas streams must allow equal amounts of gas to pass through them.
U.S. Pat. No. 3,481,703 to Zirngibl teaches the use of an inert gas heated by an electric arc as a means to heat the vapors of the metal chloride and the oxidizing gas to the reaction ignition temperature. This patent teaches flow distributions for the electrode, but does not teach or suggest anything concerning the mass or energy distribution of the reactants.
U.S. Pat. No. 3,642,442 to Hoekje et al. teaches the preparation of pigmentary titanium dioxide using electric-arc-heated oxidizing gas to contact a vapor stream of titanium tetrachloride. Parameters of operation as taught in this patent, may cause the reaction to be limited by the rate of mixing which, particularly in the case of the oxidation of iron chlorides, may result in high levels of deposits on the reactor walls.
U.S. Pat. No. 4,073,874 and U.S. Pat. No. 3,925,057 to Fukushima teach a process for oxidization of ferric chloride vapor. The '874 patent teaches that in the reactor, the velocity of the oxidizing gas, fed at the perimeter of the reactor, and the velocity of central discharge of ferric chloride vapors are selected to be within the range of Reynolds numbers that prevent the ferric chloride vapors from penetrating the surrounding flow of oxidizing gas. Both the '874 and the '057 patents provide processes that are limited by the rate of mixing.
In all of these patents listed above, the reaction zone, that is the zone or space in the reactor where the oxidation reaction takes place, extends to the reactor walls.
An object of the present invention is to provide a process for the oxidation of metal chlorides and recovery of chlorine in which the formation of wall deposits are minimized or controlled.
Another object of the present invention is to allow the direct feed of particles of metal chlorides into the reactor to avoid the need for preheating and/or vaporizing the metal chloride before it enters the reactor.
Another object of the present invention is to provide a process that does not require additional heat to be applied to the reactor beyond that required to initiate the oxidation of the metal chloride.
Another object of the present invention is to provide a process that may be operated as a stand-alone reactor or as a reactor integrated into a process generating metal chloride byproducts. In this integrated installation of the present process, chlorine recovered from the oxidation process may be, for example, recycled into a chlorinator. Solid feed of the metal chlorides, particularly iron rich chlorides, may be fed to the oxidation reactor directly from a separation process following the chlorinator.
Another object of the present invention is to provide a process in which the product oxide particles cool below their sticking point before they reach the reactor walls.
Another object of the present invention is to provide a process in which the bulk temperature is relatively low resulting in higher equilibrium conversions of the metal chloride to chlorine.